1. Field of the Invention
The present invention relates to a light emitting element array, an exposure device for exposing a photosensitive member using the light emitting element array and an image forming apparatus for forming an image through the exposure of the photosensitive member. In this type of image forming apparatus, an image is formed on a photosensitive member by exposing the photosensitive member in electrophotography manner to a light from an array of a plurality of light emitting elements.
2. Related Background Art
The formation of an image by electrophotography is a process consisting of a series of the following steps as is already known: forming a latent image by forming an optical image on a photosensitive member with a light emitting device, developing the latent image with a toner, transferring the image to a transfer material (paper), fixing it and finally cleaning the photosensitive member.
In an image forming apparatus, laser optical systems, in which a laser beam scanning is performed through a polygon mirror, are widely used as an exposure unit for forming a latent image on the surface of a photosensitive member because they have high resolution and high speed. However, since a large space is required to arrange optical parts such as a polygon mirror and lenses, it is difficult to reduce the size of this device. Further, since the scanning of the laser beam is carried out by the mechanical rotation movement of the polygon mirror, there is a limitation in increasing the operation speed.
To cope with this, much attention is paid to an array of a plurality of organic light emitting elements which does not involve the above problems as a light emitting device. Mechanical scanning is not required for the light emitting element array, scanning with an electrical signal is carried out, and such optical system as a polygon mirror and lenses are not necessary, thereby eliminating the above problems of the arrangement space of the laser optical system and limitation to the operation speed. As for concrete constitution, a plurality of light emitting elements are composed of a transparent anode layer, organic compound layer and cathode layer which are formed on a prolonged belt-like transparent substrate in the order named and arranged linearly. The organic compound layer is formed like a prolonged belt which is common to a plurality of light emitting elements and its front and rear sides are supported by a combination of predetermined cathode and anode patterns and emit light independently.
When the entire light emitting element array is formed on a single substrate, it becomes expensive. Therefore, a plurality of light emitting element chips are synthesized and disposed in an array. However, since there are differences in light emission characteristics among the plurality of chips, there is nonuniformity in the quantity of emitted light among the plurality of chips.
To solve this problem, after a light emitting element array is formed, the emitted light quantity distribution of all the elements is measured to prepare light quantity compensation data for each of the light emitting elements so as to correct the quantity of light individually with the light quantity correction circuit (for example, current correction or pulse width correction) of a drive circuit based on the data. However, there is such a problem that the drive circuit or the like becomes complex in structure.
Alternatively, it is conceivable that a drive circuit is provided in each light emitting element and a thin film resistor substrate in the drive circuit is corrected by laser trimming or the like to optimize a light emission current. However, this method cannot be adapted to a time division drive system for sharing a drive circuit to reduce costs, and hence is not generally employed.
It is an object of the present invention to attain uniformity in the quantity of light emitted from the above array of light emitting elements with a simple structure.
According to the present invention, the area of each light emitting portion is adjusted in accordance with the quantity of light emitted from each of a plurality of light emitting elements in a light emitting element array. That is, the area of the light emitting portion of a light emitting element which emits a large quantity of light is made relatively small whereas the area of the light emitting portion of a light emitting element which emits a small quantity of emitted light is made relatively large, thereby making uniform the light quantity distribution of the entire array of light emitting elements.
According to an embodiment of the present invention, the area of the light emitting portion of a light emitting element which emits a relatively large quantity of light out of a plurality of light emitting elements is selectively reduced. A reduction in the area of the light emitting portion is attained by irradiating part of the light emitting portion with an energy beam to modify the electrode or light emitting layer of the portion to prevent it from emitting light.